C,
was evaporated from a Knudsen cell onto the (100) surfaces of Si and GaAs at room temperature under ultra-high vacuum conditions. The deposition process was monitored in situ by Raman spectroscopy in the spectral region 1350-1600 cm-', where the most intense Raman features of Cm are observed.
In the initial stage of deposition on Si( IOO), the Raman spectra show a peak at 14691~~' which is soon replaced by another one at 1458 cm-'. The same is observed for deposition on GaAs( lOO), where this effect is even more pronounced. The first peak is assigned to the intrinsic pentagonal pinch mode of pure C,, while the second has been reported as the signature of the photoind&ed p&me&ation of C, which is formed under light irradiation with sufficient intensitv (-10 W/cm*). Although the intensitv in the monitoring experiment was as high as 60 W/cm', the depos;d C, film appears not-to be polymer&l at sufficiently low coverages. From the Fabry-Perot interference modulation of the Raman scattering intensity, the growth rate of the Cm film is calculated. The correlation with the evolution of Raman spectra reveals a critical thickness of about 15 nm for the onset of polymerization of the C& on both Si( 100) and GaAs( 100) under irradiation.